Phonon Excitations Research Articles

Monolayer and Bilayer Graphene on Ru(0001): Layer-Specific and Moiré-Site-Dependent Phonon Excitations.

Graphene phonons are excited by the local injection of electrons and holes from the tip of a scanning tunneling microscope. Despite the strong graphene-Ru(0001) hybridization, monolayer graphene unexpectedly exhibits pronounced phonon signatures in inelastic electron tunneling spectroscopy. Spatially resolved spectroscopy reveals that the strength of the phonon signal depends on the site of the moiré lattice with a substantial red-shift of phonon energies compared to those of free graphene. Bilayer graphene gives rise to more pronounced spectral signatures of vibrational quanta with energies nearly matching the free graphene phonon energies. Spectroscopy data of bilayer graphene indicate moreover the presence of a Dirac cone plasmon excitation.

Ultrafast Control of Material Optical Properties via the Infrared Resonant Raman Effect

The Raman effect -- inelastic scattering of light by lattice vibrations (phonons) -- produces an optical response closely tied to a material's crystal structure. Here we show that resonant optical excitation of IR and Raman phonons gives rise to a Raman scattering effect that can induce giant shifts to the refractive index and induce new optical constants that are forbidden in the equilibrium crystal structure. We complete the description of light-matter interactions mediated by coupled IR and Raman phonons in crystalline insulators -- currently the focus of numerous experiments aiming to dynamically control material properties -- by including a forgotten pathway through the nonlinear lattice polarizability. Our work expands the toolset for control and development of new optical technologies by revealing that the absorption of light within the terahertz gap can enable control of optical properties of materials over a broad frequency range.

Interfacial engineering for the enhancement of interfacial thermal conductance in GaN/AlN heterostructure

Effective heat dissipation is the bottleneck problem for the development and commercialization of GaN-based high-power electronic and photonic devices. To address this challenge and explore the underlying mechanism of phonon transportation across the GaN/AlN heterointerface, in this work, we formed three types of GaN/AlN heterostructures with distinctively different interfacial morphologies by annealing recrystallization approach. It is found that the interfacial thermal conductance (ITC) of GaN/AlN heterostructures can be remarkably improved by tailoring the interfacial crystal structure and phase morphology. Besides the commonly amorphous phase and ideal ordered wurtzite phase, we further found that AlN may present an additional stable rock salt phase at the interfacial region, and its significant effect on interfacial thermal transport has been observed. Using molecular dynamics simulation, we systematically investigated the effects of different GaN/AlN heterojunctions on the ITCs. Our results suggest that heat dissipation at the GaN/AlN interface is dominated by phonons scattered diffusely by the amorphous region at interfaces and the ITC can be significantly enhanced by recrystallizing the amorphous AlN to rock salt one. Furthermore, through phonon vibrational spectrum, we revealed that phonon modes dominate the energy transport across the interfaces of wurtzite AlN/GaN, amorphous AlN/GaN, and rock salt AlN/GaN are significantly different. Finally, we found the ITC increased with the temperature due to the enhanced inelastic phonon scattering and the presence of additional excited phonon modes at higher temperatures. The findings elucidated here provide a clearer insight into the effect of interfacial microstructures on the interfacial thermal resistance of GaN–substrate interface, which also provide a viable heat management strategy for the high-power GaN-based devices.

Probing anharmonic phonons by quantum correlators: A path integral approach.

We devise an efficient scheme to determine vibrational properties from Path Integral Molecular Dynamics (PIMD) simulations. The method is based on zero-time Kubo-transformed correlation functions and captures the anharmonicity of the potential due to both temperature and quantum effects. Using analytical derivations and numerical calculations on toy-model potentials, we show that two different estimators built upon PIMD correlation functions fully characterize the phonon spectra and the anharmonicity strength. The first estimator is associated with the force-force quantum correlators and, in the weak anharmonic regime, yields reliable zero-point motion frequencies and thermodynamic properties of the quantum system. The second one is instead connected to displacement-displacement correlators and accurately probes the lowest-energy phonon excitations, regardless of the anharmonicity strength of the system. We also prove that the use of generalized eigenvalue equations, in place of the standard normal mode equations, leads to a significant speed-up in the PIMD phonon calculations, both in terms of faster convergence rate and smaller time step bias. Within this framework, using ab initio PIMD simulations, we compute phonon dispersions of diamond and of the high-pressure I41/amd phase of atomic hydrogen. We find that in the latter case, the anharmonicity is stronger than previously estimated and yields a sizeable red-shift in the vibrational spectrum of atomic hydrogen.

Giant phonon anomalies in the proximate Kitaev quantum spin liquid \u03b1-RuCl3

The Kitaev quantum spin liquid epitomizes an entangled topological state, for which two flavors of fractionalized low-energy excitations are predicted: the itinerant Majorana fermion and the Z2 gauge flux. It was proposed recently that fingerprints of fractional excitations are encoded in the phonon spectra of Kitaev quantum spin liquids through a novel fractional-excitation-phonon coupling. Here, we detect anomalous phonon effects in α-RuCl3 using inelastic X-ray scattering with meV resolution. At high temperature, we discover interlaced optical phonons intercepting a transverse acoustic phonon between 3 and 7 meV. Upon decreasing temperature, the optical phonons display a large intensity enhancement near the Kitaev energy, JK~8 meV, that coincides with a giant acoustic phonon softening near the Z2 gauge flux energy scale. These phonon anomalies signify the coupling of phonon and Kitaev magnetic excitations in α-RuCl3 and demonstrates a proof-of-principle method to detect anomalous excitations in topological quantum materials.

Applying Unconventional Spectroscopies to the Single-Molecule Magnets, Co(PPh3 )2 X2 (X=Cl, Br, I): Unveiling Magnetic Transitions and Spin-Phonon Coupling.

Large separation of magnetic levels and slow relaxation in metal complexes are desirable properties of single-molecule magnets (SMMs). Spin-phonon coupling (interactions of magnetic levels with phonons) is ubiquitous, leading to magnetic relaxation and loss of memory in SMMs and quantum coherence in qubits. Direct observation of magnetic transitions and spin-phonon coupling in molecules is challenging. We have found that far-IR magnetic spectra (FIRMS) of Co(PPh3 )2 X2 (Co-X; X=Cl, Br, I) reveal rarely observed spin-phonon coupling as avoided crossings between magnetic and u-symmetry phonon transitions. Inelastic neutron scattering (INS) gives phonon spectra. Calculations using VASP and phonopy programs gave phonon symmetries and movies. Magnetic transitions among zero-field split (ZFS) levels of the S=3/2 electronic ground state were probed by INS, high-frequency and -field EPR (HFEPR), FIRMS, and frequency-domain FT terahertz EPR (FD-FT THz-EPR), giving magnetic excitation spectra and determining ZFS parameters (D, E) and g values. Ligand-field theory (LFT) was used to analyze earlier electronic absorption spectra and give calculated ZFS parameters matching those from the experiments. DFT calculations also gave spin densities in Co-X, showing that the larger Co(II) spin density in a molecule, the larger its ZFS magnitude. The current work reveals dynamics of magnetic and phonon excitations in SMMs. Studies of such couplings in the future would help to understand how spin-phonon coupling may lead to magnetic relaxation and develop guidance to control such coupling.

Bulk electronic structure of lanthanum hexaboride ( LaB6 ) by hard x-ray angle-resolved photoelectron spectroscopy

In the last decade rare-earth hexaborides have been investigated for their fundamental importance in condensed matter physics, and for their applications in advanced technological fields. Among these compounds, LaB$_6$ has a special place, being a traditional d-band metal without additional f- bands. In this paper we investigate the bulk electronic structure of LaB$_6$ using hard x-ray photoemission spectroscopy, measuring both core-level and angle-resolved valence-band spectra. By comparing La 3d core level spectra to cluster model calculations, we identify well-screened peak residing at a lower binding energy compared to the main poorly-screened peak; the relative intensity between these peaks depends on how strong the hybridization is between La and B atoms. We show that the recoil effect, negligible in the soft x-ray regime, becomes prominent at higher kinetic energies for lighter elements, such as boron, but is still negligible for heavy elements, such as lanthanum. In addition, we report the bulk-like band structure of LaB$_6$ determined by hard x-ray angle-resolved photoemission spectroscopy (HARPES). We interpret HARPES experimental results by the free-electron final-state calculations and by the more precise one-step photoemission theory including matrix element and phonon excitation effects. In addition, we consider the nature and the magnitude of phonon excitations in HARPES experimental data measured at different temperatures and excitation energies. We demonstrate that one step theory of photoemission and HARPES experiments provide, at present, the only approach capable of probing true bulk-like electronic band structure of rare-earth hexaborides and strongly correlated materials.

Electron backscattering coefficients of molybdenum and tungsten based on the Monte Carlo simulations

Monte Carlo simulation is employed for the calculation of electron backscattering coefficients of molybdenum (Mo) and tungsten (W) at normal incidence angle and at energies between 100 eV to 100 keV. For the modelling of the electron elastic scattering we have applied the partial-wave expansion method using the Mott's cross-section, where the scattering potential includes the electron electrostatic potential, exchange potential and correlation-polarization potential. A relativistic dielectric functional approach with full Penn's algorithm is adopted for the calculation of the electron inelastic cross-section. The effect of phonon excitation, the interband transition of the loosely bound valance electrons, and the inner-shells electrons excitations are considered in the energy loss functions of the materials via the optical constants. We found that our present simulated data agree well with the experimental data at energies above 20 keV for both elements while they are well above the experimental data at lower incident energies. The disagreement can be explained by the possible surface contamination of carbonaceous atomic layers exist in the measurements by additional simulation for the contaminated surfaces. Moreover, for the better and detailed understanding of the energy dependence of the backscattering coefficients, we also present the backscattered electron energy spectra, angular distributions, and depth distributions for both samples.

Dynamic structure factors of a strongly interacting Fermi superfluid near an orbital Feshbach resonance across the phase transition from BCS to Sarma superfluid

We theoretically investigate dynamic structure factors of a strongly interacting Fermi superfluid near an orbital Feshbach resonance with random phase approximation, and find their dynamical characters during the phase transition between a balanced conventional Bardeen-Cooper-Schrieffer superfluid and a polarized Sarma superfluid by continuously varying the chemical potential difference of two spin components. In a BEC-like regime of the BCS superfluid, dynamic structure factors can do help to distinguish the in-phase ground state from the out-of-phase metastable state by the relative location of molecular excitation and Leggett mode, or the minimum energy to break a Cooper pair. In the phase transition between BCS and Sarma superfluid, we find the dynamic structure factor of Sarma superfluid has its own specific gapless excitation at a small transferred momentum which is mixed with the collective phonon excitation, and also a relatively strong atomic excitation at a large transferred momentum because of the existence of unpaired Fermi atoms, these signals can be used to differentiate Sarma superfluid from BCS superfluid.

Enhancement of ion diffusion by targeted phonon excitation

Ion diffusion is important in a variety of applications, yet fundamental understanding of the interaction of phonons and the mobile species in solids is still missing. In this work, we introduce two formalisms that determine the individual contributions of phonons to the diffusion of ions through a solid, based on nudged elastic-band calculations and molecular dynamics simulations. The results for a model ion conductor of Ge-substituted Li 3 PO 4 (Li 3.042 Ge 0.042 P 0.958 O 4 ) reveal that more than 87% of the Li + ion diffusion originates from less than 10% of the vibrational modes between 8 and 20 THz. By deliberately exciting a small, targeted subset of these contributing modes (<1%) to a higher temperature and still keeping the lattice at a low temperature, we observe an increase in diffusivity by several orders of magnitude. This new understanding identifies new avenues for increasing diffusivity by engineering the vibrations in a material without necessarily changing the compound chemistry. Computationally detecting the contributing phonons to the ion hop in solids Identifying the important phonons in the Li + diffusion in Ge-doped Li 3 PO 4 Targeted phonon excitation raises diffusivity without raising the bulk temperature Avenue for increasing diffusivity by phonon engineering without changing chemistry Gordiz et al. computationally show that, by exciting the top contributing phonons to the ion hop, diffusivity increases without raising the bulk temperature. This observation suggests that diffusion,or more generally any transition, can be enhanced by exciting only the contributing phonons to that transition to elevated temperatures, instead of heating up the entire material.

Manipulation of Stacking Order in Td-WTe2 by Ultrafast Optical Excitation.

Subtle changes in stacking order of layered transition metal dichalcogenides may have profound influence on the electronic and optical properties. The intriguing electronic properties of Td-WTe2 can be traced to the break of inversion symmetry resulting from the ground-state stacking sequence. Strategies for perturbation of the stacking order are actively pursued for intentional tuning of material properties, where optical excitation is of specific interest since it holds the potential for integration of ultrafast switches in future device designs. Here we investigate the structural response in Td-WTe2 following ultrafast photoexcitation by time-resolved electron diffraction. A 0.23 THz shear phonon, involving layer displacement along the b axis, was excited by a 515 nm laser pulse. Pump fluences in excess of a threshold of ∼1 mJ/cm2 result in formation, with an ∼5 ps time constant, of a new stacking order by layer displacement along the b axis in the direction toward the centrosymmetric 1T* phase. The shear displacement of the layers increases with pump fluence until saturation at ∼8 pm. We demonstrate that the excitation of the shear phonon and the stabilization of the metastable phase are decoupled when using an optical pump as evidenced by observation of a transition also in samples with a pinned shear phonon. The results are compared to dynamic first-principles simulations and the transition is interpreted in terms of a mechanism where transient local disorder is prominent before settling at the atomic positions of the metastable phase. This interpretation is corroborated by results from diffuse scattering. The correlation between excitation of intralayer vibrations and interlayer interaction demonstrates the importance of including both short- and long-range interactions in an accurate description of how optical fields can be employed to manipulate the stacking order in 2-dimensional transition metal dichalcogenides.

Infrared surface phonon nanospectroscopy of an interacting dielectric-particle–dielectric-substrate dimer using fast electrons

Refinements in energy monochromation and aberration correction in state-of-the-art scanning transmission electron microscopes has opened access to the far infrared regime for spectroscopic characterization at the nanoscale. At these low energies, the dielectric environment, such as a dielectric slab adjacent to the target specimen, may no longer play a passive role in the spectrum. Instead, the environment may itself host resonances that mix with those of the target and complicate interpretation of its spectral responses. This paper explores a theoretical description of the coupling between the collective vibrational surface modes of a dielectric particle and dielectric slab of varying thickness for the purpose of elucidating the interacting phononic excitations in dielectric materials typical of inelastic electron scattering measurements in the infrared. Dynamical coordinates and a governing Hamiltonian are rigorously defined in the quasistatic limit to account for phonon mode mixing and forcing by an aloof electron probe, which travels along a grazing trajectory, parallel to the dielectric slab. As the spectral window of interrogation by fast electron probes has been extended down to thermal energies with unprecedented meV energy resolution, theoretical models like that presented herein are crucial for accurate interpretation of experimental data.

Strong Modulation of Band Gap, Carrier Mobility and Lifetime in Two-Dimensional Black Phosphorene through Acoustic Phonon Excitation.

Black phosphorene (BP) has been attracting intense attention due to its high charge mobility and potential applications in electronic, optical and optoelectronic devices. We demonstrate by ab initio molecular dynamics and nonadiabatic quantum dynamics simulations that the excitation of out-of-plane acoustic phonon (ZA) provides strong modulation of the band gap, carrier lifetime and carrier mobility in BP. A 1% tensile strain can significantly enhance ZA mode excitation at room temperature, distinctly reducing the band gap, carrier mobility, and lifetime. These electronic properties can be tuned easily by influencing the excitation amplitude of the ZA mode. Unique to the family of two-dimensional materials, the ZA mode plays an essential role in controlling the electronic properties of BP. The results of our study provide valuable guidelines for design of functional nanodevices based on 2D BP.

Coherent control of acoustic phonons in a silica fiber using a multi-GHz optical frequency comb

Multi-gigahertz mechanical vibrations that stem from interactions between light fields and matter—known as acoustic phonons—have long been a subject of research. In recent years, specially designed functional devices have been developed to enhance the strength of the light-matter interactions because excitation of acoustic phonons using a continuous-wave laser alone is insufficient. However, the strength of the interaction cannot be controlled appropriately or instantly using these structurally-dependent enhancements. Here we show a technique to control the effective interaction strength that does not operate via the material structure in the spatial domain; instead, the method operates through the structure of the light in the time domain. The effective excitation and coherent control of acoustic phonons in a single-mode fiber using an optical frequency comb that is performed by tailoring the optical pulse train. This work represents an important step towards comb-matter interactions.

Silicon carbide detectors for sub-GeV dark matter

We propose the use of silicon carbide (SiC) for direct detection of sub-GeV dark matter. SiC has properties similar to both silicon and diamond, but has two key advantages: (i) it is a polar semiconductor which allows sensitivity to a broader range of dark matter candidates; and (ii) it exists in many stable polymorphs with varying physical properties, and hence has tunable sensitivity to various dark matter models. We show that SiC is an excellent target to search for electron, nuclear and phonon excitations from scattering of dark matter down to 10 keV in mass, as well as for absorption processes of dark matter down to 10 meV in mass. Combined with its widespread use as an alternative to silicon in other detector technologies and its availability compared to diamond, our results demonstrate that SiC holds much promise as a novel dark matter detector.

Energy transfer to the phonons of a macromolecule through light pumping

In the present paper we address the problem of the energy downconversion of the light absorbed by a protein into its internal vibrational modes. We consider the case in which the light receptors are fluorophores either naturally co-expressed with the protein or artificially covalently bound to some of its amino acids. In a recent work [Phys. Rev. X 8, 031061 (2018)], it has been experimentally found that by shining a laser light on the fluorophores attached to a protein the energy fed to it can be channeled into the normal mode of lowest frequency of vibration thus making the subunits of the protein coherently oscillate. Even if the phonon condensation phenomenon has been theoretically explained, the first step - the energy transfer from electronic excitation into phonon excitation - has been left open. The present work is aimed at filling this gap.

First-principles self-consistent phonon approach to the study of the vibrational properties and structural phase transition of BaTiO3

We investigate the cubic-to-tetragonal ferroelectric phase transition temperature ${T}_{c}$ of $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ through a first-principles self-consistent phonon scheme. This method extends the harmonic approximation by incorporating anharmonic effects due to thermalized displacements, which are simultaneous excitations of phonons with a temperature-dependent random amplitude. The calculated forces are analyzed with an effective harmonic force constant model, which serves as the basis for a new set of thermalized displacements until self-consistency has been reached. The phonons with imaginary frequencies at $T=0$ K are stabilized due to anharmonic effects in the temperature range where the corresponding phase is stable and the dynamical instabilities disappear. Using the calculated free energies at various temperatures, volumes, and $c/a$ ratios, we obtain a thermal expansion in good agreement with the experimental values. Comparing the free energies of the tetragonal and cubic phases at different temperatures, we predict ${T}_{c}\ensuremath{\approx}455\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. This is in reasonable agreement with the experimental value of $\ensuremath{\approx}393$ K in view of the strong influence of the particular density functional theory approximation on the unstable phonon modes.

Collective dynamics of liquid acetone investigated by inelastic X-ray scattering

The dynamic structure factors S(Q,ω) of liquid acetone were measured in a wide Q range 1.9–41.2 nm−1 with a high-resolution inelastic X-ray scattering (IXS) spectrometer installed at BL35XU of the SPring-8. Highly damped collective excitations of the longitudinal acoustic (LA) phonons are observed. The IXS spectra were analyzed by using a generalized Langevin formalism with the help of a sparse modeling to deduce reasonable Q dependences of the fitting parameters. The dispersion relation of the LA phonon excitations shows a highly positive deviation by about 65% from the adiabatic velocity of sound, which is larger than that in non-polar liquids such as CCl4 and benzene with van der Waals interactions, due probably that the acetone is a polar liquid. Microscopic dynamic parameters, such as viscoelastic relaxation rates and viscosity, are discussed by comparing with existing macroscopic dynamic properties. The fast and slow viscoelastic relaxations correspond to the vibration dynamics and reorientation correlations, respectively.

Dynamics of Photoexcited Small Polarons in Transition-Metal Oxides.

The dynamics of photoexcited polarons in transition-metal oxides (TMOs), including their formation, migration, and quenching, plays an important role in photocatalysis and photovoltaics. Taking rutile TiO2 as a prototypical system, we use ab initio nonadiabatic molecular dynamics simulation to investigate the dynamics of small polarons induced by photoexcitation at different temperatures. The photoexcited electron is trapped by the distortion of the surrounding lattice and forms a small polaron within tens of femtoseconds. Polaron migration among Ti atoms is strongly correlated with quenching through an electron-hole (e-h) recombination process. At low temperature, the polaron is localized on a single Ti atom and polaron quenching occurs within several nanoseconds. At increased temperature, as under solar cell operating conditions, thermal phonon excitation stimulates the hopping and delocalization of polarons, which induces fast polaron quenching through the e-h recombination within 200 ps. Our study proves that e-h recombination centers can be formed by photoexcited polarons, which provides new insights to understand the efficiency bottleneck of photocatalysis and photovoltaics in TMOs.

Surface-Phonon-Induced Rotational Dissipation for Nanoscale Solid-State Gears

Compared to nanoscale friction of translational motion, the mechanisms of rotational friction have received less attention. Such motion becomes an important issue for the miniaturization of mechanical machinery that often involves rotating gears. In this study, molecular-dynamics simulations are performed to explore rotational friction for solid-state gears rotating on top of different substrates. In each case, viscous damping of the rotational motion is observed and found to be induced by the pure van der Waals interaction between the gear and the substrate. The influence of different gear sizes and various substrate materials is investigated. Furthermore, the rigidities of the gear and the substrate are found to give rise to different dissipation channels. Finally, it is shown that the dominant contribution to the dissipation is related to the excitation of low-frequency surface phonons in the substrate.